Typical prior art switch mode power converters have a supply input and an output that provides a regulated supply voltage. U.S. Pat. No. 5,600,234 shows a DC-DC buck converter with a switching cell that transforms an input voltage to a regulated supply voltage lower than the input voltage. The switching cell is controlled by a pulse signal that has a fixed period and a variable duty cycle. The pulse signal is provided by a summing comparator. The summing comparator has a first differential input pair to which a saw-tooth waveform signal is applied that determines the fixed period of the pulse signal. The duty cycle of the pulse signal is set by a first feedback loop that feeds the a fraction of the voltage at the output of the switching cell to an inverting input of the summing comparator, and by a second feedback loop that includes an integrating differential amplifier. While the first feedback loop ensures a fast transient response, it also introduces a DC error. The second feedback loop has a high gain but slow transient response to correct for the DC error and provide a stable steady-state operating point.